The present invention generally relates to a spread-spectrum (communication) system of a direct sequencing spread spectrum modulation type and more particularly to a spread-spectrum communication system in which synchronism between a transmitter and a receiver is held in a satisfactory manner.
In the field of modern industrial machines, progress is made in systematization. There prevails a tendency that controls are allocated in a dispersed manner and/or supervised centrally. Under the circumstance, importance is put on the information or data transmitting path more and more. As an example of such system, there may be mentioned a communication system destined for controlling movable machines which can move freely.
In the communication system of the type mentioned above, adoption of wire communication is not preferred in applications where the movable machines are allowed to move around over a wide area. On the other hand, in case the movable machines are moved in a region where plants are operated, the use of electromagnetic wave of high energy level for the data transmission is undesirable because disturbance may then be given to electronic instruments and equipments of the plants.
Such being the circumstances, a spread-spectrum communication system is reconsidered which permits transmission by electromagnetic wave of relatively low energy level and is excellent in the noise insusceptibility.
According to the spread-spectrum communication system, a data signal to be transmitted is converted into a signal having a broader bandwidth than the bandwidth that the data signal has inherently before being transmitted, whereby communication of high quality can be realized. More particularly, in the transmitter, a carrier modulated with a specific code which is called a pseudo noise code and has a sufficiently broader bandwidth than the information or data signal is modulated with the data signal again and transmitted. On the other hand, in the receiver, the received signal is demodulated with a spectrum spreading code which is same as the pseudo noise code used in the transmitter, to thereby receive selectively the transmitted data signal.
The spread-spectrum communication is valid on the condition that the spreading code used in the receiver perfectly coincides in phase with the spreading code contained in the received signal.
According to the literature (R. C. Dixon, Tateno, Latest Spectrum Spread Communication) published Oct. 25, 1981 this synchronization for synchronizing phase and frequency needs much time. An example of a synchronizing circuit for the phase matching or coincidence is disclosed in Japanese Laid-Open Patent Application No. 31,246/1981.
However, the prior art spread-spectrum communication system requires two carriers in one way communication.